The results challenge the assumption that human evolution followed a path from a chimplike ancestor to a transitionary Homo erectus and then Homo sapiens, suggesting instead that chimpanzees have more in common developmentally with Homo erectus and that modern humans are the "out-group."

The study was coauthored by Adrienne Zihlman, professor of anthropology at the University of California, Santa Cruz; Debra Bolter, who just earned her doctorate in anthropology at UCSC; and Christophe Boesch, director of primatology at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The researchers examined skeletal samples from 18 wild chimpanzees of known ages and compared the data to the dentition of captive chimpanzees, which have been used as a baseline for discussion of hominid origins and the transition from ape ancestors to hominids. The eruption of teeth mark other life events, such as completion of brain growth (90 to 95 percent of brain growth is complete when the first permanent molar erupts) and life-history stages like infancy, juvenile, and adulthood.

The team's analysis consistently showed a slower rate of development of all the teeth of wild chimpanzees compared to captive chimpanzees: Among wild chimpanzees, infancy lasted until about four years of age and mature dentition was reached between 12 and 13 years of age, compared to captive animals whose infancy ended around three years of age and who reached mature dentition about 10 years of age.

"These findings challenge a number of assumptions about the growth of hominids," said Zihlman. "Anthropologists and paleoanthropologists have relied heavily on studies of captive chimpanzees to establish a baseline for hominid growth and to generate hypotheses about the life history and behaviors of fossil humans. We now know those scenarios are based on faulty data."

Comparing teeth of wild chimpanzees to previous research on two Homo erectus fossil specimens, the researchers found that the first molars of both wild chimpanzees and Homo erectus emerge at about four years of age, and the second molars emerge at about eight years of age.

"Our data suggest that wild chimpanzees and Homo erectus growth patterns may not have differed from each other as much as previously thought," said Zihlman. "These findings do not support Homo erectus developmentally as an intermediate between chimplike ancestors and modern humans. Our data also call into question the assumption that a larger body size and a big brain require a longer time to grow."

The findings also explain why dental-eruption data derived from captive chimpanzees didn't match the life stages of wild animals observed by researchers in the field, Zihlman noted.

The skeletal samples examined included 12 immature individuals and one young adult from the Tai National Park, Ivory Coast, four immatures from Gombe National Park in Tanzania, and one immature from Bossou, Guinea.

The paper, "Wild Chimpanzee Dentition and its Implications for Assessing Life History in Immature Hominin Fossils," is scheduled to appear in the July 20 print edition of the Proceedings of the National Academy of Science.

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Editor's Note: Adrienne Zihlman can be reached at (831) 459-4467 or via e-mail at azihlman@ucsc.edu.